In general, the invention relates to optimising the use of a radio interface in a radio system. In particular, the invention relates to the controlling of transmission power with the aim of utilizing the radio interface in an efficient way. Here the term radio system refers particularly to a cellular radio system where the mobile stations can communicate with various base stations.
A cellular radio system has a given amount of radio resources at its disposal. These resources can be described in a coordinate system where the coordinates are frequency, time and location, among others. In other words, in each area, there are certain radio frequencies which are available for a certain period of time. In order to maximise the data transmission capacity in the system and to minimise the power consumption of portable terminals, it is extremely important that the radio resources are utilised in an optimal fashion.
In the CDMA (Code Division Multiple Access) system, where there are several simultaneous radio connections at the same frequency, separated by means of code division, the transmission power is an important factor when optimising the use of radio resources. Particularly important the choice of transmission power is in a so-called macrodiversity connection, i.e. in a situation where in between the mobile station and the network, essentially identical data is transmitted through at least two different base stations. In a correctly arranged macrodiversity connection, the transmission power can be kept so low that the interference caused for other simultaneous connections remains lower than in a case where the connection between a mobile station and the network is arranged via one base station only. A poorly arranged macrodiversity connection may multiply the interference and remarkably reduce the total capacity of the system. The present patent application deals with both macrodiversity connections and conventional connections passing through one base station only.
In a prior art CDMA system, there is often applied so-called two-level power control. An outer control loop, i.e. so-called quality loop, tries to find a suitable target level for the SIR (Signal to Interference Ratio), the BER (Bit Error Ratio) and/or the FER (Frame Error Ratio) of the connection, and/or for some other factor describing the quality of the connection in question. An inner control loop attempts to adjust the transmission power so that the latest target level(s) reported by the outer control loop should be achieved. In order to compensate fast fade-outs and the so-called near-far phenomenon, the inner control loop operates very rapidly, even thousands of times per second. A typical rate of operation for the inner control loop is 1600 times per second. This type of control loop functions so that a receiving device investigates whether the SIR value or some other factor describing the connection quality surpasses the target level or falls under it, and gives feedback to that effect to the transmitting device. In the simplest form, said feedback is only a command to increase or reduce transmission power, in which case it can be expressed in one bit: for instance the bit value zero is interpreted as a command to reduce transmission power, and the bit value one is interpreted as a command to increase transmission power. A given step, for example 1 dB, for increasing or reducing transmission power, can be agreed on in advance.
Let us first observe uplink data transmission in a macrodiversity connection, where the mobile station transmits and the base stations receive. Each base station measures, for its own part, a SIR value or the like, compares it with the target level and sends a power control command as feedback to the mobile station. The mobile station surveys the received power control commands and applies an algorithm in order to decide whether it should reduce or increase its transmission power. A simple algorithm functions so that the mobile station increases its transmission power, if it receives from all base stations a command for increasing transmission power, and reduces its transmission power, if it receives a command to that effect from even one base station. Other algorithms can be, and are, used.
In downlink data transmission, the mobile station compares the measured SIR value or the like with the target level and sends, on the basis of the result obtained in this comparison, a power control command which is received by all base stations operating in said macrodiversity connection.
In addition to power control commands, also other so-called control information is transmitted in between the mobile stations and the base stations. Power control commands and other control information differ from user data or the actual data to be transmitted in that their contents are not meant for the information of the user, but they are used to control factors linked with the use and functionality of the connection. In addition to power control commands, another example of control information are the RI (Rate Information) bits included in each frame to be transmitted over each radio connection and used for sending information from the transmitting to the receiving device about the data rate related to the frame in question. A third example of control information are the pilot bits used in channel estimation. Such parts of the frame where control information is transmitted can be called control fields. There can be set various requirements for each control field as to how reliably the control information contained therein must be correctly understood in the receiver.
In a prior art arrangement, a problem results in that data transmission in between the mobile station and the base stations is not faultless, in which case the receiving device can misinterpret the control information sent by the transmitting device. If for instance the power control command is one (possibly repetition encoded) bit, its value may change to the opposite owing to interference occurring in the radio connection, in which case the device which should have its transmission power adjusted misinterprets the command for increasing the transmission power and reduces its transmission power, or vice versa. In general, it can be assumed that the probability to misinterpret the received control information is a decreasing function of the channel quality. The channel quality is described for instance by the SIR value.
The object of the present invention is to introduce a method and a system whereby the problems related to the reception of control information can be reduced both in a macrodiversity connection and in a connection between one mobile station and one base station. Another object of the invention is that the implementation of the methods and arrangements according to the invention does not require an unreasonable amount of signalling in between the fixed network facilities or in between base stations and mobile stations. Yet another object of the invention is that radio resources can be efficiently utilised thereby.
These objects of the invention are achieved by providing the base stations and/or mobile stations with a possibility to transmit the information belonging to the control fields (for example pilot field, power control field and RI field bits) at a different power than the actual data to be transmitted. Each control field may have its own transmission power, which is defined either as an absolute power value or as a power difference between it and some other field. In addition, there must be created a suitable, versatile power control system, which controls the transmission power of the data associated with the control channel. Said data associated with the control channel here refers particularly to the power control commands related to the control of the inner loop, but also other data associated with the physical arrangement of the radio connection.
The method according to the invention is applicable for controlling the transmission power in a data transmission system comprising a first device and a second device and a two-way data transmission connection in between, wherein the first device sends to the second device user data and the second device sends to the first device user data and control information. It is characterised in that it comprises the steps of
determining a first transmission power level, a second transmission power level and a third transmission power level and
transmitting from a second device to a first device a frame of data by applying said first transmission power level to the transmission of a user data field, said second transmission power level to the transmission of a first control information field and said third transmission power level to the transmission of a second control information field.
The invention also relates to a mobile station and base station in a cellular radio system, said stations being characterised in that they are provided to function as the first or second device in the method described above.
According to the invention, the reliability of the control information in the reception can be adjusted by sending the bits belonging to a given control field at a higher or lower power than the bits belonging to some other field of the same frame. When an optimal transmission power is found for the control field bits, the reliability of the control information in the reception has reached the desired level, and the total interference in the system remains as low as possible. Naturally an increase in the transmission power improves reliability in the reception, and respectively a decrease in the transmission power weakens reliability.
The transmission power of the control field bits is most advantageously chosen according to how reliable the receiving device interprets them to be, or how much quality-weakening interference there is assumed to occur in the radio connection. On the other hand, the transmission power of the different parts of the control information can also be adjusted depending on the measured connection quality in between the receiving and transmitting device, in the same or in the opposite transmission direction. For instance, the transmission power of the power control commands related to a downlink connection can be changed with respect to the transmission power of the rest of the bits contained in the same frame, if the quality of the uplink connection is unnecessarily good or too bad. In similar fashion, the transmission power of the power control commands related to a downlink connection with respect to the transmission power of the rest of the bits contained in the same frame can be reduced, if the quality of the uplink connection conforms strictly to the target level. In that case, the connection quality is typically described with a SIR value.
The increasing of the transmission power of the above mentioned RI bits with respect to the transmission power of the rest of the bits contained in the same frame improves the reliability of the RI bit interpretation performed by the receiver, in which case the receiver can, with a higher probability, handle the data bits belonging to the frame correctly. The power difference between the pilot bits and the data bits can be adjusted on the basis of how good is the value of the quantity describing the connection quality to which the pilot bits are in average connected during a given measurement period. The increase of the pilot bit transmission power aims mainly at improving the reliability of the channel estimate (and the SIR estimate).
The controlling of the transmission power separately for each control field improves the efficiency in the utilisation of radio resources, because an unnecessarily high power is not used for transmitting such control commands and other control information that can be received to a sufficiently reliable degree even when transmitted at a lower power.